1. Technical Field
The invention pertains to methods of fabricating display screens. In particular embodiments, the invention pertains to methods of reducing precipitate formation in a phosphor-containing solution during electrophoretic deposition of phosphor, and to methods of depositing phosphor over selected regions of a substrate.
2. Description of the Related Art
Phosphor-containing display devices have numerous applications, including, for example, utilization as TV screens and computer monitors. Phosphor-containing display devices generally utilize one or more components to project electrons against a phosphor to cause one or both of fluorescence or phosphorescence, and to thereby cause an image to be displayed. Exemplary components which can be utilized to generate electrons are cathode ray tubes, and cathode emitter arrays.
An exemplary phosphor-containing display device 40 is described with reference to FIG. 1. Device 40 is a field emission display (FED) device comprising a plurality of phosphor molecules 33 (only some of which are labeled) coated over a conductive layer 34, which in turn is over a transparent display screen 35. The phosphor molecules can also be referred to as xe2x80x9cphosphorxe2x80x9d. Conductive layer 34 can comprise, for example, indium tin oxide, and transparent screen 35 can comprise, for example, glass. Screen 35 can be referred to as a face plate. Device 40 further comprises a base plate 12 spaced from face plate 35, and which can also comprise glass. A conductive layer 14 is over base plate 12, and can comprise, for example, conductively-doped semiconductive material.
Emitters 26 are formed over and in electrical connection with conductive material 14. Dielectric regions 28 (only some of which are labeled) and an emitter grid 30 (only some of which is labeled) are formed over layer 14 and proximate emitters 26. Insulative spacers 32 are provided to support face plate 35 in a spaced relation relative to base plate 12. A power source 37 is provided to supply a voltage differential between conductive layer 34, conductive layer 14, and emitter grid 30.
In operation, cathode emitters 26 are electrically stimulated to cause electrons 36 (shown as dashed lines, and only some of which are labeled) to be ejected from emitters 26 and against phosphor molecules 33. The electrons then cause one of both of phosphorescence and fluorescence by phosphor molecules 33 to result in an image being displayed. Such image can be viewed by a user looking through transparent face plate 35.
The individual phosphor molecules 33 can all comprise a single uniform color (such as, for example, green) or can comprise a multitude of colors, depending on the application. Frequently, three colors of phosphor molecules 33 (for instance, red, green and blue) are provided. Each of the three colors is formed in a specific region separate from the others of the three colors, and the specific regions are surrounded by black regions.
Methodology for forming phosphor-coated face plate 35 is described with reference to an electrophoretic deposition system 70 illustrated in FIG. 2. System 70 comprises an electrophoretic deposition bath 50 contained within a vessel 51. Glass plate 35, having conductive layer 34 formed thereover, is placed within electrophoretic deposition bath 50. Conductive material 34 is utilized as a first electrode within bath 50, and a second electrode 52 is also provided within bath 50. A power source 53 is provided to electrically charge electrodes 34 and 52, with electrode 34 being charged as a negative electrode and electrode 52 being charged as a positive electrode.
Bath 50 typically comprises a mixture of isopropyl alcohol, glycerol and water, within which phosphor particles and metal complexes are dissolved. Additional electrolyte ions, besides the phosphor particles and metal ions of the metal complexes, can also be dissolved within solution 50. An exemplary solution 50 comprises 80 milligrams of isopropyl alcohol (99.5% pure), 0.35 grams of phosphor, 0.2 grams of glycerol (100% pure), and 0.025 grams of one or both of In(NO3)2 and Ce(NO3)3.H2O.
In operation, power applied from source 53 generates a negative potential at conductive layer 34 which attracts positively charged ions 63 to a surface of conductive layer 34. The positively charge ions comprise phosphor molecule ions and metal ions. The negative potential at conductive layer 34 also causes hydrolysis of water to form hydroxide ions adjacent the surface of conductive layer 34. The hydroxide ions and metal ions interact with the phosphor particle surface to form a complex which adheres to surface 34.
In particular applications, conductive material 34 can be formed in a pattern over face plate 35 as shown in FIG. 3. Such pattern leaves some portions 37 of face plate 35 uncovered with conductive material 34. Since the phosphor molecule ions selectively deposit on conductive material 34, the patterning of conductive material 34 shown in FIG. 3 can result in the phosphor molecules being deposited in a pattern corresponding to the pattern of conductive material 34 over face plate 35. Portions 37 of glass plate 35 between regions of conductive material 34 can be either covered with a protective layer (such as, for example, photoresist) prior to the electrophoretic deposition of phosphor molecules, or left exposed to the deposition conditions.
After the electrophoretic deposition described with reference to FIG. 2, face plate 35 is removed from deposition bath 50 and rinsed with isopropyl alcohol. Such rinsing preferably leaves the complexes of phosphor molecule ions, metal ions and hydroxide ion over conductive material 34, while removing phosphor particles from regions where the particles are unintended to be deposited.
After the rinsing, face plate 35 is dried by, for example, thermal dehydration or infrared radiation dehydration.
In embodiments in which multiple colors of phosphor molecules are to be deposited over a single face plate, the electrophoretic deposition and rinsing will be repeated for each color of phosphor molecule that is to be deposited. For instance, if red, green and blue phosphor molecules are to be deposited on a glass substrate, a first lithography, first electrophoretic deposition and subsequent isopropyl rinse will be done with one of the three colors of phosphor molecules, and subsequently a second and third lithography, electrophoretic deposition and isopropyl rinse will be done with each of the remaining two colors of phosphor molecules.
The processing described above with reference to FIGS. 2 and 3 has difficulties associated therewith. For instance, a precipitate forms over time within electrophoretic deposition bath 50 which complicates repeated utilization of the deposition bath. Further, it is found that phosphor molecules deposited over conductive material 34 will occasionally be displaced by the isopropyl alcohol rinse to cause bleeding of phosphor colors and to reduce a total amount of phosphor ultimately formed over conductive material 34. It would be desirable to develop methodologies which overcome one or both of the above-described difficulties.
In one aspect, the invention encompasses a method of fabricating a display screen. Phosphor molecules are electrophoretically deposited onto a substrate in a first solution having a first concentration of hydroxide ions. The deposited phosphor molecules are then rinsed with a second solution having a second concentration of hydroxide ions greater than the first concentration of hydroxide ions.
In another aspect, the invention encompasses a method of depositing phosphor molecules over selected regions of a substrate. Conductive regions are formed over portions of a substrate while non-conductive regions are left over other portions of the substrate. Phosphor molecules are electrophoretically deposited onto the conductive regions of the substrate in a first solution having a first concentration of hydroxide ions. The substrate is then rinsed with a second solution having a second concentration of hydroxide ions. The second concentration of hydroxide ions is greater than the first concentration of hydroxide ions. After the rinsing, the deposited phosphor molecules are dried.
In yet another aspect, the invention encompasses a method of forming an FED device.